Heterophil/Lymphocyte Ratio Level Modulates Resistance, Cecal Microbiota Composition and Functional Capacity in Infected Chicken.

The gastrointestinal microbiota plays a vital role in ensuring the maintenance of host health through interactions with the immune system. The Heterophil/Lymphocyte (H/L) ratio reflects poultry's robustness and immune system status. Chickens with low H/L ratio are superior to the chickens with high H/L ratio in survival, immune response, and resistance to infection, but the underlying mechanisms remain unclear. This study aimed to identify microorganisms associated with resistance to Enteritidis infection in chickens based on the H/L ratio. The 16S rRNA and metagenomic analysis were conducted to examine microbiome and functional capacity between the 2 groups, and Short Chain Fatty Acids (SCFAs) and histopathology were conducted to explore the potential difference between susceptible and resistant groups at 7 and 21 days post-infection (dpi). The microbiome exploration revealed that low H/L ratio chickens, compared to high H/L ratio chickens, displayed a significantly higher abundance of () and () at 7 and 21 dpi, respectively. (r = 0.63) and (r = 0.63) were identified as bacterial genus significantly correlated with H/L (P < 0.001). Interestingly, was significantly and positively correlated with bodyweight post-infection (r = 0.72), propionate (r = 0.78) and valerate (r = 0.82) contents, while was significantly and negatively correlated with bodyweight post-infection (r = - 0.67), propionate (r = - 0.61) and valerate (r = - 0.65) contents (P < 0.001). Furthermore, the comparative analysis of the functional capacity of cecal microbiota of the chickens with high and low H/L ratio revealed that the chickens with low H/L ratio possess more enriched immune pathways, lower antibiotic resistance genes and virulence factors compared to the chickens with high H/L ratio. These results suggest that the chickens with low H/L ratio are more resistant to Enteritidis, and it is possible that the commensal and are involved in this resistance against infection. These findings provide valuable resources for selecting and breeding disease-resistant chickens.